\documentclass[border=5pt, tikz]{standalone}

\usepackage{tikz}
% \usetikzlibrary{positioning}

\begin{document}
\begin{tikzpicture}
% \tikzstyle{shorten <>}=[shorten >=#1, shorten <=#1]
% \tikzset{shorten <>/.style={shorten >=#1, shorten <=#1}}
\node [draw=green, text=red] (a) {A};
% \node [draw, shape=circle, fill=cyan!30, text=red] (b) [right=3em of a] {B};
\node [draw, shape=circle, fill=cyan!30, anchor=west] (b) at ([xshift=3em]b.east) {B};
% \draw [-stealth, shorten <>=2pt, blue] (a) -- (b);
\draw [-stealth, blue] (a) -- (b);
% \node [fill=brown, circle, inner sep=0pt, minimum size=4pt, anchor=north west] at (b.south east) {};
\end{tikzpicture}
\end{document}